It is well known to utilize an electronic sensor to detect a condition through an opening in a surface, such as a wall or duct. U.S. Pat. No. 5,922,939 discloses a humidity sensor that can be mounted on an outside surface of a duct to detect a level of humidity within the duct. The humidity sensor includes a mounting enclosure attached to an outside of the duct, and a sleeve extending from the mounting enclosure and through an aperture formed in the duct. A sensing unit is mounted within the sleeve and is thereby positioned within the duct for sensing the level of humidity within the duct. The sleeve is releaseably attached to the mounting enclosure via a swage nut so that the sleeve and sensing unit can be removed from the duct for servicing without the need to remove the mounting enclosure. The sensing unit can then be removed from the sleeve for service or replacement with a like unit. The repaired or replaced sensing unit is then installed into the sleeve, and the sleeve is then repositioned into the duct and reattached to the mounting enclosure with the swage nut.